Commitments and Contingencies (Insurance) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($)
Insurance [Abstract]
Maximum obtainable amount of private liability insurance available under Price-Anderson Act	$ 450,000,000
Amount of secondary financial protection liability insurance coverage per incident	13,000,000,000
Potential retrospective assessments under secondary financial protection system	1,000,000,000
Potential retrospective assessments under secondary financial protection system payable per incident per year	152,000,000
Amount of coverage per occurrence per site for property damage, decontamination and premature decommissioning risks	2,750,000,000
Amount of sublimit for non nuclear perils per occurrence per site under nuclear insurance mutual companies for property damage decontamination and premature decommissioning risks	$ 1,500,000,000
Coinsurance, percent	10.00%
Coinsurance, limit of coverage per loss per site occurrence	$ 400,000,000
Potential retrospective assessment, limited insurance coverage per occurrence per site, nuclear insurance mutual companies, property damage decontamination and premature decommissioning risks	178,000,000
Seabrook Station Insurance [Member]
Insurance [Abstract]
Potential retrospective assessment recoverable from minority interest for nuclear liability secondary financial protection	15,000,000
Potential retrospective assessment recoverable from minority interest for property damage, decontamination and premature decommissioning risks	2,000,000
Duane Arnold Energy Center Insurance [Member]
Insurance [Abstract]
Potential retrospective assessment recoverable from minority interest for nuclear liability secondary financial protection	38,000,000
Potential retrospective assessment recoverable from minority interest for property damage, decontamination and premature decommissioning risks	5,000,000
St Lucie Unit No 2 Insurance [Member]
Insurance [Abstract]
Potential retrospective assessment recoverable from minority interest for nuclear liability secondary financial protection	19,000,000
Potential retrospective assessment recoverable from minority interest for property damage, decontamination and premature decommissioning risks	4,000,000
FPL[Member]
Insurance [Abstract]
Potential retrospective assessments under secondary financial protection system	509,000,000
Potential retrospective assessments under secondary financial protection system payable per incident per year	76,000,000
Potential retrospective assessment, limited insurance coverage per occurrence per site, nuclear insurance mutual companies, property damage decontamination and premature decommissioning risks	108,000,000
Duane Arnold Energy Center Insurance [Member]
Insurance [Abstract]
Amount of sublimit for non nuclear perils per occurrence per site under nuclear insurance mutual companies for property damage decontamination and premature decommissioning risks	$ 1,000,000,000